Polarization-direction-controlling element and exposure device

ABSTRACT

A polarization-direction-controlling element comprising a ½ wavelength plate disposed with a crystal optical axis tilted at substantially 45 degrees with respect to a polarization direction of a beam of light separated by a polarization-separating element with a part of a laser beam transmitted, is provided on the optical path of the laser beam, outputted from a plurality of semiconductor lasers, between an outlet for the laser beams at a fiber array and a polarization-separating element for separating the laser beam into two beams of light having mutually orthogonal polarization directions. A polarization-direction-controlling element capable of improving the quality of recorded images in an exposing-recording device using an element with polarization dependency and an exposure device capable of improving the quality of recorded images can also be obtained.

This is a divisional of application Ser. No. 10/180,075 filed Jun. 27,2002 now U.S. Pat. No. 6,967,671. The entire disclosure of the priorapplication, application Ser. No. 10/180,075, is hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a polarization-direction-controllingelement and an exposure device. More specifically, it relates to apolarization-direction-controlling element and an exposure device usedin an exposing-recording device for forming an image by scanning arecording medium with light outputted from a light source.

2. Description of the Related Art

An exposing-recording device for recording a two-dimensional image on aphotosensitive material by rotating a drum with the photosensitivematerial (recording medium) mounted on the outer circumferential surfacein a main scanning direction and directing a laser beam modulatedaccording to image data of an image to be recorded on the photosensitivematerial for scanning in a sub scanning direction orthogonal to the mainscanning direction, has been conventionally used.

In this kind of the exposing-recording device, in order to record animage with a low resolution, a method in which a spot size of the laserbeam on the photosensitive material surface is reduced and a recordingpitch in the sub scanning direction is widened, or a pixel of the sameimage data is repeatedly recorded for reduction of the resolutionwithout change in the spot size or the recording pitch. In contrast, inorder to record an image with a high resolution, an opposite method isused.

In order to enlarge or reduce the laser beam spot size, however, a lensof an optical system, or the like should be driven by a drivingmechanism, and thus there is a problem in that the device becomes bulkyand the costs are increased. Moreover, when a pixel of the same imagedata is repeatedly recorded in order to reduce the resolution, therecording pitch in the sub scanning direction is constant, and thusthere is a problem in that a recording speed cannot be improved.

Therefore, in order to solve these problems, according to the techniquedisclosed in the official gazette of the Japanese Patent ApplicationLaid-open (JP-A) No. 2000-284206 by the present inventor, aplural-focal-point-producing means for dividing a beam of lightoutputted from a light source into a plurality of beams of light and forproducing a plurality of focal points on a recording medium with respectto the sub scanning direction of the recording medium by using alight-condensing optical system, and a sub-scanning-controlling meansfor controlling the recording interval in the sub scanning directionaccording to the resolution are provided. As a result, the number of thefocal points, which are produced by the plural focal point producingmeans by division of the beam of light in the sub scanning directionaccording to a desired resolution of the recorded image at the time ofimage recording by condensing the light outputted from the light sourceon the recording medium via the light-condensing optical system, iscontrolled in order to adjust the size of the beam spot and to adjustthe recording interval of the beam spot in the sub scanning direction.This enables efficient recording of an image at the desired resolution.

In order to improve recording speed, there is an exposing-recordingdevice in which laser beams outputted from a plurality of light sourcesare each guided to a single exposure head by an optical fiber and inwhich laser-beam outlets, which are each disposed at an end of one ofthe optical fibers at the exposure head, are provided side by side forsimultaneously executing the exposure by the plurality of the laserbeams outputted from the plurality of the light sources.

When the above-mentioned technique disclosed in JP-A No. 2000-284206 isused in this kind of the exposing-recording device, exposure with alaser beam divided into a plurality of laser beams is possible, and thusfurther improvement in recording speed can be achieved.

However, according to the above-mentioned exposing-recording deviceusing the optical fibers, the polarization direction of the lightoutputted from the optical fibers may change over time due todisplacement by external forces applied to the optical fibers (includingvibration, pressure, and distortion), temperature displacement, or thelike as shown in FIG. 14. In this case, since the light is dividedunevenly among the plurality of beams so as to make the focal spotsunstable, there is a problem in that the quality of the recorded imageis deteriorated.

In other words, as shown in FIG. 15A as an example, in a case where twofocal points are produced by the above-mentioned plural focal pointproducing means by using as the light source a semiconductor laser, withan intensity distribution having a high central light intensity, and thelight intensity is gradually lowered as it moves away from the center,it is ideal in terms of image quality, to have the intensitydistribution of the two resulting beams of light correspond to the twofocal points in the same state as shown in FIG. 15B.

In a case where the laser beam polarization direction is changed overtime as mentioned above, however, since there is a risk of theintensities of the two resulting beams of light corresponding to thefocal points becoming drastically different as shown in FIG. 15C, theimage quality of the recorded image may be deteriorated. According to anexperiment conducted by the present inventor, it was found that thepolarization ratio of the horizontal polarization and the verticalpolarization (horizontal polarization:vertical polarization) is changedin a range of 1:4 to 4:1 when an optical-fiber-coupled semiconductorlaser is used as a light source.

SUMMARY OF THE INVENTION

In order to solve the above-mentioned problems, the present inventionhas been achieved, and a first object thereof is to provide apolarization-direction-controlling element capable of improving theimage quality of a recorded image in an exposing-recording device usingan element having the polarization dependency, and a second object is toprovide an exposure device capable of improving the image quality of arecorded image.

In order to achieve the above-mentioned first object, a first aspect ofthe invention is a polarization-direction-controlling element to beprovided at an upstream side, in an optical-axis direction, of a beam oflight in an exposure device using a polarization-separating element forseparating the beam of light into two beams of light having mutuallyorthogonal polarization directions, thepolarization-direction-controlling element comprising: a plate-likebase, having two mutually parallel flat surfaces for transmitting thebeam of light, one of the two surfaces being an incident surface towhich the beam of light is irradiated, and the other being an outputsurface for outputting the light, and a plurality of ½ wavelength platesprovided on at least one of the incident surface and the output surfaceof the base such that a crystal optical axis of each of the ½ wavelengthplates is tilted at an angle within a predetermined range, whichincludes 45 degrees, with respect to a polarization direction of thebeam of light to be separated by the polarization-separating element.The above-mentioned crystal optical axis is referred to also as the“optical axis”, however, in this specification, it is disclosed as the“crystal optical axis”. Furthermore, although the predetermined anglerange including 45 degrees is ideally 45 degrees, it denotes an angle invarious tolerance ranges such as an angle in the tolerance range in theproduction of the polarization-direction-controlling element, an anglein the tolerance range in a device using thepolarization-direction-controlling element, or the like.

Here, with reference to FIG. 13, the principal of the invention will beexplained. A case of the polarization-direction-controlling element ofthe invention provided as a combination of a ½ wavelength plate and atransparent parallel plate without a drastic influence on thepolarization direction of a transmitted light will be described.

As shown in FIG. 13, with the premise that the polarization direction ofthe light to be separated by the polarization-separating element is (x,y), the polarization-direction-controlling element is disposed so as tohave the direction of the crystal optical axis of the ½ wavelength platein the polarization-direction-controlling element is tilted by 45degrees with respect to (x, y). The coordinate system of the crystaloptical axis in the ½ wavelength plate is defined to be (X, Y).Moreover, with the premise that the light transmissivity of thetransparent parallel plate and that of the ½ wavelength plate in thepolarization-direction-controlling element have no difference, or it isso small that it can be ignored, the polarization-direction-controllingelement is disposed at a position such that the ratio of the amount oflight incident on the ½ wavelength plate and the amount of light not tobe incident, that is, a light incident on the transparent parallel plateis 1:1.

With the premise that the electric field vector α of a light incident onthe polarization-direction-controlling element is a α=(a, b), inconsideration of a light incident on the ½ wavelength plate, a matrix Afor rotating the (x, y) coordinate system by 45 degrees, a matrix B fordelaying the light of the Y coordinate by a ½ wavelength phase and amatrix C for rotating by −45 degrees to the original (x, y) coordinatesystem can be represented as follows.

$\begin{matrix}{{\alpha = \begin{pmatrix}a \\b\end{pmatrix}}{A = {\begin{pmatrix}{\cos\; 45{^\circ}} & {\sin\; 45{^\circ}} \\{{- \sin}\; 45{^\circ}} & {\cos\; 45{^\circ}}\end{pmatrix} = {\frac{\sqrt{2}}{2}\begin{pmatrix}1 & 1 \\{- 1} & 1\end{pmatrix}}}}{B = {\begin{pmatrix}1 & 0 \\0 & {\mathbb{e}}^{{- i} \cdot \pi}\end{pmatrix} = \begin{pmatrix}1 & 0 \\0 & {- 1}\end{pmatrix}}}{C = {\begin{pmatrix}{\cos\; 45{^\circ}} & {{- \sin}\; 45{^\circ}} \\{\sin\; 45{^\circ}} & {\cos\; 45{^\circ}}\end{pmatrix} = {\frac{\sqrt{2}}{2}\begin{pmatrix}1 & {- 1} \\1 & 1\end{pmatrix}}}}} & (1)\end{matrix}$

Therefore, the electric field vector β of the light after passingthrough the ½ wavelength plate can be represented as follows.

$\begin{matrix}{\beta = {{C \cdot B \cdot A \cdot \alpha} = \begin{pmatrix}a \\b\end{pmatrix}}} & (2)\end{matrix}$

Since α and β are set so as to be 1:1 as the amount of lightdistribution, each light amount Iα, Iβ are represented as follows.

$\begin{matrix}{{{I\;\alpha} = {\begin{pmatrix}I & \alpha & x \\I & \alpha & y\end{pmatrix} = {\frac{\alpha^{2}}{2} = {\frac{1}{2}\begin{pmatrix}a^{2} \\b^{2}\end{pmatrix}}}}}{{I\;\beta} = {\begin{pmatrix}I & \beta & x \\I & \beta & y\end{pmatrix} = {\frac{\beta^{2}}{2} = {\frac{1}{2}\begin{pmatrix}a^{2} \\b^{2}\end{pmatrix}}}}}} & (3)\end{matrix}$

Therefore, the amount of light I as the summation of the lights isrepresented as follows.

$\begin{matrix}{I = {\begin{pmatrix}I & x \\I & y\end{pmatrix} = {{{I\;\alpha} + {I\;\beta}} = {\frac{1}{2}\begin{pmatrix}{a^{2} + b^{2}} \\{a^{2} + b^{2}}\end{pmatrix}}}}} & (4)\end{matrix}$

This result denotes that in the case the light transmitted the ½wavelength plate and the light not transmitted are added, the lightseparated by the polarization-separating element into the x and ypolarization directions is separated by the equal light amount.

The transparent parallel plate provided here in thepolarization-direction-controlling element need not always provided, andanother member not having a significant influence on the polarizationdirection of an incident light (such as an ND (Neutral Density) filter)can be used instead of the transparent parallel plate, or it is alsopossible to have a configuration in which these members are notprovided.

According to the above-mentioned principal, thepolarization-direction-controlling element of the first aspect of theinvention can separate an amount of light into the equal amounts by thepolarization-separating element in the case of use in a combination witha polarization-separating element so that the image quality of therecorded image can be improved in an exposing-recording device using thepolarization-separating element.

In order to separate a light into equal amounts by thepolarization-separating element as mentioned above, it is necessary thatthe amount of light is distributed so as to have the ratio of the amountof light incident on the ½ wavelength plate and the amount of light notincident thereon to be 1:1 in the condition that the lighttransmissivity of the transparent parallel plate in thepolarization-direction-controlling element (or a member provided insteadof the transparent parallel plate such as the above-mentioned ND filter,or a part without providing anything) and that of the ½ wavelength plateis same or so small to an ignorable degree.

Then, a second aspect of the invention is thepolarization-direction-controlling element according to the firstaspect, wherein the area ratio of an area of the ½ wavelength plate tohave the light incident thereon and an area of the light not to beincident on the ½ wavelength plate is substantially 1:1. Thereby, theintensity distribution of the separated light can be evened. The “areaof the light not to be incident on the ½ wavelength” here corresponds tothe area of the above-mentioned transparent parallel plate with thelight incident thereon, the area of the member instead of thetransparent parallel plate, such as the ND filter with the lightincident thereon, or the area of a portion where nothing is providedwith the light incident thereon.

According to the second aspect, although it is advantageous in terms ofevening the intensity distribution of a separated light on the conditionthat the light transmissivity of the transparent parallel plate (or themember instead of the transparent parallel plate such as theabove-mentioned ND filter, or the part without providing anything) andthat of the ½ wavelength plate are same or small to an ignorable degree,in the case the difference of the light transmissivity is not in anignorable degree, the separated light intensity distribution can hardlybe evened.

Then, a third aspect of the invention is the first aspect, wherein thearea ratio of an area of the ½ wavelength plate to have the lightincident thereon and an area of the light not to be incident on the ½wavelength plate is provided such that the amount of lights of the twobeams of light obtained by the polarization-separating element aresubstantially same. Thereby, the intensity distribution of the separatedlight can certainly be evened. The “area of the light not to be incidenton the ½ wavelength” here also corresponds to the area of theabove-mentioned transparent parallel plate with the light incidentthereon, the area of the member instead of the transparent parallelplate, such as the ND filter with the light incident thereon, or thearea of the portion where nothing is provided with the light incidentthereon.

As a specific embodiment of the third aspect of the invention, in thecase the light transmissivity ratio ½ wavelength plate:passage area ofthe light not incident on the ½ wavelength plate) in the polarizationdirection of the light separated by the polarization-separating elementof the ½ wavelength plate and the passage area of the light not incidenton the ½ wavelength plate (the above-mentioned transparent parallelplate, the member instead of the transparent parallel plate such as theND filter, or the part without providing anything)=1:η, the ratio of thearea H of the ½ wavelength plate with the light incident thereon and thearea S with the light not incident on the ½ wavelength plate can be setas represented by the following formula.(H:S)=η:1

Moreover, according to a fourth aspect of the invention, apolarization-direction-controlling element is provided, which maycomprise a plurality of ½ wavelength plates disposed at predeterminedintervals in the entire incident area of the light.

Furthermore, according to a fifth aspect of the invention, apolarization-direction-controlling element for controlling thepolarization direction of an incident light is provided, wherein a ½wavelength plate is disposed so that the amount of lights of thetransmitted light and the light not transmitted is substantially same.

As a specific method for having the substantially same light amount atthe time, in addition to the method of adjusting the area ratio of thearea of the ½ wavelength plate with the light incident thereon and thearea of the light not incident on the ½ wavelength plate, a method ofadjusting the amount of light transmitted the ½ wavelength plate and thelight not transmitted by using at least one selected the groupconsisting of an AR coating (Antireflection coating) and an ND filtercan be used.

Furthermore, a polarization-direction-controlling element according to asixth aspect of the invention is configured by attaching the ½wavelength plate on a transparent parallel plate.

The above-mentioned transparent parallel plate need not alwayscompletely transparent or have a 100% light transmissivity as long as itdoes not drastically change the polarization direction of the incidentlight.

On the other hand, in order to achieve the above-mentioned secondaspect, a seventh aspect of the invention provides an exposure devicecomprising a light source for outputting a beam of light, alight-condensing optical system for condensing the beam of lightoutputted from the light source onto a recording medium, apolarization-separating element for separating the beam of light intotwo beams of light having mutually orthogonal polarization directions,and the polarization-direction-controlling element according to any ofthe first to sixth aspects, disposed between the light source and thepolarization-separating element, with the crystal optical axis of the ½wavelength plate tilted at an angle within a predetermined range, whichincludes 45 degrees, with respect to a polarization direction of thebeam of light separated by the polarization-separating element.

According to the exposure device of the seventh aspect of the invention,at the time the light outputted from the light source is collected onthe recording medium by the light-condensing optical system, the lightis separated into the two beams of light with the polarizationdirections orthogonal with each other by the polarization-separatingelement. The above-mentioned light source includes various kinds ofsemiconductor lasers. Moreover, the above-mentionedpolarization-separating element includes various kinds of prisms, suchas a Rochon Prism and a Wollaston Prism.

Furthermore, although the predetermined angle range including 45 degreesis ideally 45 degrees, it denotes an angle in various tolerance rangessuch as a tolerance range in the exposure device of the invention.

According to the exposure device of the seventh aspect of the invention,a light can be separated by the equal amount by thepolarization-separating element so that the quality of the image can beimproved at the time of recording an image on a recording medium by theseparated lights.

An exposure device of an eighth aspect of the invention is the exposuredevice according to the seventh aspect, wherein thepolarization-separating element is for separating the beam of light intotwo beams of light that include an ordinary ray and an extraordinaryray.

Here, according to a ninth aspect of the invention, thepolarization-separating element is provided at a position where thelight is a parallel light flux for outputting the two beams of lightwith different angles so that the light can be separated.

Moreover, according to a tenth aspect of the invention, the light can beseparated by providing the polarization-separating element at a positionwhere the light diverges or position where the light converges so as tooutput the two beams of light from different positions with respect tothe light separation direction of the polarization-separating element.

Furthermore, according to an eleventh aspect of the invention, atransfer section is further provided for transferring thepolarization-separating element so as to be inserted on the optical axisof the light or removed there from so that the resolution can be changedeasily at the time of recording the image on the recording medium byinserting or removing the polarization-separating element by the movingsection with respect to the optical path.

Moreover, according to a twelfth aspect of the invention, a transfersection is further provided for transferring thepolarization-direction-controlling element and thepolarization-separating element so as to be inserted to the optical axisof the beam of light or removed therefrom simultaneously so that theresolution can be changed easily at the time of recording the image onthe recording medium by inserting or removing thepolarization-direction-controlling element and thepolarization-separating element by the moving means with respect to theoptical path.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic configuration diagram (plan view) of a laserrecording device 10A according to a first embodiment of the presentinvention.

FIG. 2 is a schematic configuration diagram (side view) of a fiber arraypart 30 according to the first embodiment of the invention.

FIG. 3 is a schematic diagram (plan view) for explaining theconfiguration and the function of a polarizations separating element 36according to the first embodiment of the invention.

FIG. 4A is a schematic configuration diagram (plan view and side view)of a polarization-direction-controlling element 34 according to thefirst embodiment of the invention; and FIG. 4B is a schematic diagramshowing the crystal optical axis direction of thepolarization-direction-controlling element 34 according to the firstembodiment of the invention.

FIGS. 5A and 5B are schematic diagrams showing the separation state of alaser beam by the polarization-separating element 36 in the case withoutusing the polarization-direction-controlling element 34 in the laserrecording device 10 according to the first embodiment of the invention.

FIGS. 6A and 6B are schematic diagrams showing the separation state of alaser beam by the polarization-separating element 36 in the case ofusing the polarization-direction-controlling element 34 in the laserrecording device 10 according to the first embodiment of the invention.

FIG. 7 is a block diagram showing the configuration of a controllingsystem of the laser recording device 10 according to the firstembodiment of the invention.

FIG. 8 is a flow chart showing the flow of a process in the case ofrecording an image according to the resolution.

FIGS. 9A and 9B are schematic diagrams showing the state of a beam spoton a recording film F by the laser recording device 10 according to thefirst embodiment of the invention.

FIG. 10 is a schematic configuration diagram (plan view) of a laserrecording device 10B according to a second embodiment of the invention.

FIG. 11 is a schematic diagram (plan view) for explaining the functionof a polarization-separating element 36′ according to the secondembodiment of the invention.

FIG. 12A is a schematic configuration diagram (front view) showinganother configuration example of a polarization-direction-controllingelement of the invention; and FIG. 12B is a schematic diagram showinganother arrangement state of a polarization-direction-controllingelement according to the invention.

FIG. 13 is a schematic diagram for explaining the principal of theinvention.

FIG. 14 is a schematic diagram for explaining the problems in theconventional technique.

FIGS. 15A-15C are graphs showing examples of the state of the intensitydistribution of a laser beam at the focal point.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, with reference to the drawings, embodiments of the presentinvention will be explained in detail. In the description below, thecase of using a polarization-direction-controlling element and anexposure device according to the invention in a laser recording devicewill be explained.

First Embodiment

First, with reference to FIG. 1, the configuration of the laserrecording device 10A according to the first embodiment will beexplained. As shown in the figure, the laser recording device 10Aaccording to the first embodiment comprises three or more odd number of(in this embodiment, seven) semiconductor lasers LD, each for outputtinga laser beam, an exposure head 12 for condensing the laser beamsoutputted from each semiconductor laser LD, a drum 14 with a recordingfilm F for recording an image mounted, to be rotated and driven so as tomove the recording film F in the main scanning direction, and a subscanning motor 16 for moving an exposure head 12 disposed on a ballscrew 18 in the sub scanning direction orthogonal to the main scanningdirection by rotating and driving the ball screw 18. In this embodiment,as the semiconductor lasers LD, an optical fiber coupled semiconductorlaser with the intensity distribution shown in FIG. 15A is used.

In contrast, in the exposure head 12, a fiber array section 30 isprovided for outputting the laser beams guided by the above-mentionedodd number of the semiconductor lasers LD collectively so that the laserbeam outputted from each semiconductor laser LD is guided to the fiberarray section 30 by each optical fiber 20. In this embodiment, amultiple mode optical fiber having a relatively large core size is usedas the optical fiber 20 for having a high laser beam power.

FIG. 2 shows the configuration of the fiber array section 30 viewed fromthe arrow B direction in FIG. 1. As shown in the figure, the fiber arraysection 30 according to this embodiment is provided with a base 30Ahaving V-shaped grooves of the same number as that of the semiconductorlasers LD formed on the upper surface along the sub scanning directionadjacent with each other, and one each optical fiber 20 fitted in theV-shaped grooves. Therefore, a plurality of laser beams L outputted fromeach semiconductor laser LD are provided from the fiber array section 30along the sub scanning direction per a predetermined interval.

Moreover, in the exposure head 12, a collimator lens 32, apolarization-direction-controlling element 34, a polarization-separatingelement 36 and a light collecting lens 38 are arranged successively fromthe fiber array section 30 side.

Furthermore, the exposure head 12 is provided with an element movingmotor 40 for inserting the polarization-separating element 36 on theoptical path of the laser beam L or removing therefrom by rotationaround the rotation axis in the arrow A direction in FIG. 1.

As shown in FIG. 3, the polarization-separating element 36 according tothis embodiment is a Rochon prism produced by attaching two uniaxialcrystals 36A, 36B with the crystal optical axes orthogonal with eachother, for separating a laser beam L into a normal ray and an abnormalray with respect to the sub scanning direction of the recording film F.For example, the crystal optical axis of the uniaxial crystal 36Adisposed on the laser beam L incident side is set parallel with theoptical axis of the laser beam L, and the crystal optical axis of theone 0061 is crystal 36B disposed on the laser beam L output side is setin the direction orthogonal to the optical axis of the laser beam L andthe sub scanning direction. In this case, the normal ray moves straightin the polarization-separating element 36, and the abnormal ray 36 isbent by the polarization-separating element 36 in the sub scanningdirection.

As the polarization-separating element 36, a Wollaston prism having thecrystal optical axis of the uniaxial crystal 36A which is set so as tobe orthogonal to the optical axis of the laser beam L and the subscanning direction, and the crystal optical axis of the uniaxial crystal36B which is set so as to be orthogonal to the optical axis of the laserbeam L and the sub scanning direction, can be used as well.

Moreover, the polarization-separating element 36 need not alwaysseparate the laser beam L always into the normal ray and the abnormalray. Another one capable of separating the same into two beams of lightwith different polarization directions can be used.

In contrast, as shown in FIG. 4A and FIG. 4B, thepolarization-direction-controlling element 34 according to thisembodiment comprises a glass plate 34B as the base and a ½ wavelengthplate 34A disposed on the upstream side in the optical axis direction ofthe polarization-separating element 36 such that a part of the laserbeam L is transmitted as well as the crystal optical axis is tiltedsubstantially by 45 degrees with respect to the polarization directionof the light to be separated by the polarization-separating element 36.Moreover, here, the polarization-direction-controlling element 34 isprovided by bonding a plurality of the ½ wavelength plates 34A on theglass plate 34B by each both end parts (in the part not having the laserbeam L transmission) by a predetermined interval such that the ratio ofthe areas of the ½ wavelength plate 34A and the glass plate 34B forhaving the incident light can be substantially 1:1.

Here, the polarization-direction-controlling element 34 of thisembodiment is designed so as to have an optical fiber 20 with about a 50μm to 60 μm core size, about a 20 mm beam size of the laser beam L inthe light incident surface and about a 2 mm arrangement pitch interval D(see the side view of FIG. 4A) in the light incident surface of the ½wavelength plate 34A and the glass plate 34B. Thereby, the amount oflight of the laser beam incident on the ½ wavelength plate 34A and theamount of light of the laser beam directly incident on the glass plate34B can be substantially equal to each other.

Accordingly, the polarization-direction-controlling element 34 of thisembodiment comprises a plurality of the ½ wavelength plates 34A by apredetermined interval D in the entire laser beam L incident area suchthat the ratio of the area of the ½ wavelength plate 34A with the laserbeam L incident thereon and the area of the laser beam L not incident onthe ½ wavelength plate 34A, that is, the area of the glass plate 34Bwith the laser beam L incident thereon can be substantially 1:1.However, it is also possible to provide the ½ wavelength plate 34A andthe glass plate with the ratio of the areas with the laser beam Lincident thereon so as to have the amount of lights of the two laserbeams obtained by the polarization-separating element 36 besubstantially the same based on the device specification of the laserrecording device 10A, or the like. Moreover, it is also possible toadjust the position of the ½ wavelength plate 34A so as to have theamount of lights of the laser beam L transmitted the ½ wavelength plate34A and the laser beam L not transmitted (that is, the laser beam Ltransmitted only the glass plate 34B) can be substantially the same.Since the configuration of the polarization-direction-controllingelement is substantially the same as the one shown in FIG. 4, thepolarization-direction-controlling element is not further shown inanother figure.

Here, the separation state of the laser beam by thepolarization-separating element 36 in the case of not using thepolarization-direction-controlling element 34 with theabove-mentionedconfiguration, and in the case of using the same will beexplained. First, with reference to FIG. 5, the laser beam separationstate in the case of not using the polarization-direction-controllingelement 34 will be explained.

As shown in FIG. 5A, for example, in the case a laser beam of an e1+e2polarization (laser beam with a polarization tilted by 45 degrees withrespect to the polarization e1) is incident onthe-polarization-separating element 36, it is separated evenly into twopolarization e1 and polarization e2.

In contrast, as shown in FIG. 5B, in the case a laser beam of thepolarization e1 is incident on the-polarization-separating element 36,only a laser beam of the polarization e1 is outputted from thepolarization-separating element 36. Therefore, in this case, it isdifficult to separate this evenly into two laser beams.

Next, with reference to FIGS. 6A and 6B, the laser beam separation statein the case of using the polarization-direction-controlling element 34will be explained.

As shown in FIG. 6A, in the case a laser beam of an e1+e2 polarization(laser beam with a polarization tilted by 45 degrees with respect to thepolarization e1) is incident on the glass plate 34B, the laser beamtransmits the same without changing the polarization direction so thatthe transmitted laser beam is incident on the polarization-separatingelement 36 so as to be separated evenly into two polarization e1 andpolarization e2. In contrast, in the case a laser beam of an e1+e2polarization is incident on the ½ wavelength plate 34A, since thecrystal optical axis is tilted by 45 degrees with respect to thepolarization e1, the laser beam is transmitted without changing thepolarization direction so that the transmitted laser beam is incident onthe polarization-separating element 36 so as to be separated evenly intotwo polarization e1 and polarization e2.

In contrast, as shown in FIG. 6B, in the case a laser beam of the e1polarization is incident on the glass plate 34B, the laser beam istransmitted without changing the polarization direction so that thetransmitted laser beam is incident on the polarization-separatingelement 36 so as to output a laser beam of only the e1 polarization fromthe element 36. In contrast, in the case a laser beam of the e1polarization is incident on the ½ wavelength plate 34A, since thecrystal optical axis is tilted by 45 degrees with respect to thepolarization e1, the laser beam is outputted as a polarization e2 whichis orthogonal to the polarization e1 so that the outputted laser beam isincident on the polarization-separating element 36 so as to output alaser beam of only the polarization e2 from the element 36. Therefore,the laser beam can be divided equally as a whole at different positionsin the space.

Next, with reference to FIG. 7, the configuration of the controllingsystem of the laser recording device 10A according to this embodimentwill be explained. As shown in the figure, the controlling systemcomprises an LD driving circuit 54 for driving the semiconductor laserLD according to image data, an element moving motor driving circuit 56for driving the element moving motor 40, a sub scanning motor drivingcircuit 58 for driving the sub scanning motor 16, an LD driving circuit54 and a controlling circuit 52 for controlling the element moving motordriving circuit 56 and the sub scanning motor driving circuit 58. Here,to the controlling circuit 52, image data showing an image to berecorded on the recording film F and the resolution for recording theimage will be supplied.

The glass plate 34B corresponds to the transparent parallel plate of theinvention, the semiconductor laser LD corresponds to the light source ofthe invention, the collimator lens 32 and the light collecting lens 38correspond to the light-condensing optical system of the invention, andthe element moving motor 40 corresponds to the moving section of theinvention, respectively.

Next, the operation of the laser recording device 10A with theabove-mentioned configuration will be explained with reference to theflow chart of FIG. 8. In the description below, explanation will begiven with the premise that the scanning line pitch interval in the subscanning direction on the high resolution side of the recording film Fby each laser beam L in the case of not disposing thepolarization-separating element 36 on the optical path of the laser beamL, that is, the case of not separating the laser beam L is defined to beε, the beam spot interval is set at 2·ε and the beam spot displacementamount by the polarization-separating element 36 on the recording film Fby the two laser beams separated by the polarization-separating element36 is set at ε.

First, the operator inputs resolution data showing the resolution of animage to be recorded in the laser recording device 10A (step 100). Theresolution data and the image data of the image to be recorded aresupplied to the controlling circuit 52. The recording circuit 52supplies a signal adjusted based on the data to the LD driving circuit54, the element moving motor driving circuit 56 and the sub scanningmotor driving circuit 58. In this embodiment, explanation will be givenbelow with the premise that an image can be recorded by two kinds ofresolutions of R (dpi) and 2·R (dpi) as the above-mentioned resolution.

In the case that the resolution inputted by the operator is 2·R (dpi)(in the case the judgment in the step 102 is positive), the elementmoving motor driving circuit 56 drives the element moving motor 40 formoving the polarization-separating element 36 so as not to dispose thepolarization-separating element 36 on the optical path of the laser beamL (step 104). Moreover, in this case, the sub scanning motor drivingcircuit 58 sets the feeding interval W in the sub scanning direction ofthe exposure head 12 by the sub scanning motor 16 as follows (step 106).

$\begin{matrix}{W = {{\frac{\left( {N - 1} \right) \times {2 \cdot ɛ}}{2} + ɛ} = {N \cdot ɛ}}} & (5)\end{matrix}$

Therein, N represents the number of the semiconductor lasers LD (in thisembodiment “7”).

That is, in the case that the resolution is 2·R (dpi), by disposing thepolarization-separating element 36 out of the optical path of the laserbeam L, the laser beam L is not separated into two laser beams (normalray and abnormal ray) in the sub scanning direction. Thereby, theresolution of double as much as that of the case of separating the laserbeam L is realized.

When the polarization-separating element 36 is moved and the feedinginterval in the sub scanning direction is set as mentioned above, the LDdriving circuit 54 controls the drive of each semiconductor laser LDaccording to the image data (step 108).

The laser beam L outputted from each semiconductor laser LD is made tobe parallel light fluxes by the collimator lens 32 so as to be incidenton the polarization-direction-controlling element 34. As to the laserbeams L incident on the polarization-direction-controlling element 34,those incident beams on the glass plate 34B are outputted withoutchanging the polarization direction. Moreover, as to the laser beamsincident on the ½ wavelength plate 34A, those with the polarizationdirection identical with the crystal optical axis of the ½ wavelengthplate 34A are outputted without changing the polarization direction andthose beams with the polarization direction not identical with thecrystal optical axis are outputted with the polarization direction whichis changed to the direction according to the angle formed by thepolarization direction and the crystal optical axis.

The laser beams L outputted from the polarization-direction-controllingelement 34 are collected via the light collecting lens 38 onto therecording film F on the drum 14.

In this case, beam spots S1 to S7 (see FIG. 9A) having the intensitydistribution shown in FIG. 15A are formed on the recording film F. Asshown in FIG. 9A, according to the beam spots S1 to S7, atwo-dimensional image with a 2·R (dpi) resolution is formed on therecording film F by feeding the exposure head 12 in the sub scanningdirection by the feeding interval W pitch and rotating the drum 14 inthe main scanning direction (step 110).

Next, the case the resolution is changed from 2·R (dpi) to R (dpi) (inthe case the judgment in the step 102 is negative) will be explained. Inthis case, the element driving motor driving circuit 56 drives theelement moving motor 40 for moving the polarization-separating element36 so as to be disposed on the optical path of the laser beam L (step112). Moreover, in this case, the sub scanning motor driving circuit 58sets the feeding interval W′ in the sub scanning direction of theexposure head 12 by the sub scanning motor 16 as follows (step 114).W′=N×2·ε  (6)

That is, in the case that the resolution is R (dpi), by disposing thepolarization-separating element 36 on the optical path of the laser beamL, the laser beam L incident on the polarization-separating element 36is separated into two laser beams (normal ray and abnormal ray) in thesub scanning direction. Thereby, the resolution of half as much as thatof the case of not separating the laser beam L is realized.

When the polarization-separating element 36 is moved and the feedinginterval in the sub scanning direction is set as mentioned above, the LDdriving circuit 54 controls the drive of each semiconductor laser LDaccording to the image data (step 108).

The laser beam L outputted from each semiconductor laser LD is made tobe parallel light fluxes by the collimator lens 32 so as to be incidenton the polarization-direction-controlling element 34. As to the laserbeams L incident on the polarization-direction-controlling element 34,those beams incident on the glass plate 34B are outputted withoutchanging the polarization direction. Moreover, as to the laser beamsincident on the ½ wavelength plate 34A, those beams with thepolarization direction identical with the crystal optical axis of the ½wavelength plate 34A are outputted without changing the polarizationdirection and those beams with the polarization direction not identicalwith the crystal optical axis are outputted with the polarizationdirection changed to the direction according to the angle formed by thepolarization direction and the crystal optical axis.

The laser beams L outputted from the polarization-direction-controllingelement 34 are supplied to the polarization-separating element 36 fortransmitting both the normal ray and the abnormal ray. The normal rayand the abnormal ray are collected via the light collecting lens 38 ontothe recording film F on the drum 14.

In this case, beam spots S1′ to S7′ (see FIG. 9B) having the dualintensity distributions shown in FIG. 15B, that is, the intensitydistribution of the normal ray and the intensity distribution of theabnormal ray synthesized in the sub scanning direction, are formed onthe recording film F.

As shown in FIG. 9B, according to the beam spots S1′ to S7′, atwo-dimensional image with a R (dpi) resolution is formed on therecording film F by feeding the exposure head 12 in the sub scanningdirection by the feeding interval W′ pitch and rotating the drum 14 inthe main scanning direction (step 110).

Accordingly, in the case that the resolution of the recorded image ischanged from 2·R (dpi) to R (dpi), since the beam spots S1 to S7 caneasily be enlarged to the beam spots S1′ to S7′ by only inserting thepolarization-separating element 36 on the optical path of the laser beamL. Furthermore, since the sub scanning speed can be made higher, animage can be recorded at a high speed.

Similarly, the resolution can be changed from R (dpi) to 2·R (dpi).

Second Embodiment

Although a case, in which the polarization-separating element of theinvention is disposed at a position where a light is a substantiallyparallel light flux so as to output two beams of light y differentangles, has been explained in the above-mentioned first embodiment,another case, in which the polarization-separating element is disposedat a position where a light is dispersed so as to output two beams oflight from different positions with respect to the light separationdirection by the polarization-separating element, will be explained inthe second embodiment.

First, with reference to FIG. 10, the configuration of a laser recordingdevice 10B according to the second embodiment will be explained. Thesame components in the figure as those in the laser recording device 10Ashown in FIG. 1 are provided with the same numerals as those in FIG. 1and further explanation is not given here.

As shown in the figure, the laser recording device 10B according to thesecond embodiment differs from the laser recording device 10A accordingto the first embodiment in that a polarization-separating element 36′made of an uniaxial crystal is used instead of thepolarization-separating element 36 and thepolarization-direction-controlling element 34 and thepolarization-separating element 36′ are disposed at a position where thelaser beam L is dispersing between the fiber array section 30 and thecollimator lens 32. In this case, the direction of the crystal opticalaxis of the polarization-separating element 36′ is set between theoptical axis direction of the laser beam L and the sub scanningdirection.

In the case that the resolution is set at R (dpi) the laser beam L withthe polarization direction controlled by thepolarization-direction-controlling element 34 is separated into a normalray and an abnormal ray by the polarization-separating element 36 asshown in FIG. 11. In this case, since the refractive index of thepolarization-separating element 36′ with respect to the normal ray isconstant regardless of the direction of the crystal optical axis, it isemitted from an imaginary light emitting point Po on the optical axis ofthe laser beam L so as to be guided to the collimator lens 32. Incontrast, since the refractive index of the polarization-separatingelement 36′ with respect to the abnormal ray differs depending on thelaser beam L incident direction and the crystal optical axis direction.The crystal optical axis is set between the laser beam L optical axisdirection and the sub scanning direction, therefore, it is outputtedfrom the imaginary light emitting point Pe displaced from the laser beamL optical axis by a predetermined amount in the sub scanning directionso as to be guided to the collimator lens 32.

As a result, the normal ray and the abnormal ray are respectivelycollected at a position on the recording film F displaced by thepredetermined amount in the sub scanning direction via the collimatorlens 32 and the light collecting lens 38, so as to obtain the beam spotsS1′ to S7′ shown in FIG. 9D. Thereby, an image of the R (dpi) resolutioncan be formed.

In contrast, in the case of forming an image of the 2·R (dpi)resolution, the beam spots S1 to S7 shown in FIG. 9A can be obtained bymoving the polarization-separating element 36 off the optical path ofthe laser beam L.

As heretofore explained in detail, according to thepolarization-direction-controlling element 34 of the above-mentionedembodiments, since the polarization-direction-controlling elementprovided on the upstream side along the optical axis direction of thelaser beam L from the polarization-separating element 36 for separatingthe laser beam L into two laser beams having polarization directionsorthogonal to each other is provided on the upstream side along theoptical axis direction from the polarization-separating element 36 fortransmitting a part of the laser beam L and the ½ wavelength plate 34Ais disposed with the crystal optical axis tilted by substantial 45degrees with respect to the light polarization direction of the lightseparated by the polarization-separating element 36, the laser beam Lcan be separated by the equal light amount by thepolarization-separating element 36 in the case of using in a combinationwith the polarization-separating element 36 so that the image quality ofthe recorded image can be improved in the laser recording device usingthe polarization-separating element 36.

Moreover, according to the polarization-direction-controlling element 34of the above-mentioned embodiments, since the ratio of the area of the ½wavelength plate 34A with the laser beam L incident thereon and the areaof the laser beam L not incident on the ½ wavelength plate 34A, that is,the area of the glass plate 34B with the laser beam L directly incidentthereon can be substantially 1:1, the intensity distribution of theseparated laser beam can be evened.

Furthermore, according to the polarization-direction-controlling element34 of the above-mentioned embodiments, since a plurality of the ½wavelength plates 34A are provided on the entire laser beam L incidentarea at predetermined intervals, it can be produced easily.

Moreover, according to the laser recording devices 10A, 10B of theabove-mentioned embodiments, since thepolarization-direction-controlling element 34 as mentioned above isdisposed with the crystal optical axis of the ½ wavelength plate 34Atilted by substantially 45 degrees with respect to the polarizationdirection of the laser beam L separated by the polarization-separatingelement 36 between the semiconductor laser LD and thepolarization-separating element 36, the laser beam L can be separated bythe equal light amount by the polarization-separating element 36 so thatthe image quality of the image at the time of recording an image on therecording film F by the separated laser beam L can be improved.

Moreover, according to the laser recording devices 10A, 10B of theabove-mentioned embodiments, since the element moving motor 40 formoving the polarization-separating element 36 for inserting and removingthe polarization-separating element 36 on the optical axis of the laserbeam L or removing therefrom is provided, the resolution at the time ofrecording an image on the recording film can easily be changed byinserting and removing the polarization-separating element 36 on theoptical axis or removing therefrom by the element moving motor 40.

Although the case of using the polarization-direction-controllingelement 34 (see FIG. 4A) provided by attaching a plurality of flatplate-like ½ wavelength plates 34A on a glass plate 34B by apredetermined interval as the polarization-direction-controlling elementof the invention has been explained in the above-mentioned embodiments,the invention is not limited thereto, and for example, as shown in FIG.12A, an embodiment provided by attaching a plurality of cylindrical ½wavelength plates with the crystal optical axis tilted by substantially45 degrees with respect to the polarization direction of the lightseparated by the polarization-separating element with respect to acolumnar glass plate, having different sizes onto the concentric circlesof the above-mentioned glass plate can be adopted as well. Also in thiscase, the same effect as in the above-mentioned embodiments can beachieved.

Moreover, although the case of the polarization-direction-controllingelement 34 with a plurality of ½ wavelength plates 34A arranged in thesub scanning direction has been explained in the above-mentionedembodiments, the invention is not limited thereto, and for example, asshown in FIG. 12B, an embodiment with the arrangement direction which isorthogonal to the sub scanning direction can be adopted as well. Also inthis case, the same effect as in the above-mentioned embodiments can beachieved.

Furthermore, although the case of adopting the invention in the laserrecording devices 10A, 10B for executing the multiple beam scanning hasbeen explained in the above-mentioned embodiments, the invention is notlimited thereto, and for example, it can be adopted in a laser recordingdevice for executing the single beam scanning, comprising onesemiconductor laser as the light source. Also in this case, the sameeffect as in the above-mentioned embodiments can be achieved.

Moreover, although the case of providing the element moving motor 40 foronly inserting the polarization-separating element 36 on the opticalpath of the laser beam L or removing therefrom has been explained in theabove-mentioned embodiments, the invention is not limited thereto, andthe element moving motor 40 can be provided for simultaneously insertingthe polarization-direction-controlling element 34 and thepolarization-separating element 36 on the optical path of the laser beamor removing therefrom. Also in this case, the same effect as in theabove-mentioned embodiments can be achieved.

Furthermore, although the case of disposing the polarization-separatingelement 36′ at a position for dispersing the laser beam has beenexplained in the above-mentioned second embodiment, the invention is notlimited thereto, and for example, it can be disposed at a position wherethe laser beam is condensed, that is, between the light collecting lens38 and the recording film F. Also in this case, the same effect as inthe above-mentioned second embodiment can be achieved.

1. An exposure device comprising: a light source for outputting a beamof light, a light-condensing optical system for condensing the beam oflight outputted from the light source onto a recording medium; apolarization-separating element for separating the beam of light intotwo beams of light having mutually orthogonal polarization directions;and a polarization-direction-controlling element disposed between thelight source and the polarization-separating element and including ½wavelength plate, wherein a crystal optical axis of the ½ wavelengthplate is tilted at an angle within a predetermined range, which includes45 degrees, with respect to a polarization direction of the beam oflight separated by the polarization-separating element.
 2. The exposuredevice of claim 1, wherein the polarization-separating element is forseparating the beam of light into two beams of light that include anordinary ray and an extraordinary ray.
 3. The exposure device of claim1, wherein the polarization-separating element is disposed at a positionwhere the light is a substantially parallel light flux and outputs thetwo beams of light at different angles.
 4. The exposure device of claim1, wherein the polarization-separating element is provided at one of aposition where the beam of light diverges and a position where the beamof light converges, and outputs the two beams of light from differentpositions with respect to the light-separation direction of thepolarization-separating element.
 5. The exposure device of claim 1,further comprising a transfer section for transferring thepolarization-separating element so as to be inserted to and removed fromthe optical axis of the beam of light.
 6. The exposure device of claim1, further comprising a transfer section for transferring thepolarization-direction-controlling element and thepolarization-separating element so as to be inserted to the optical axisof the beam of light or removed therefrom simultaneously.